1. Field of the Invention
The present invention relates to an active matrix light emitting device using a light emitting element having a film containing an organic compound (hereinafter referred to as an organic compound layer) between a pair of electrodes which provides fluorescence or phosphorescence when applied an electric field by a TFT (Thin Film Transistor). It is to be noted that a light emitting device in this specification refers to an image display device, a light emitting device, or a light source. Further, a module in which a light emitting element is provided with a connector, for example an FPC (Flexible Printed Circuit), a TAB (Tape Automated Bonding) tape, a TCP (Tape Carrier Package), a module in which the tip of a TAB tape or a TCP is provided with a printed wiring board, and a module in which an IC (Integrated Circuit) is directly mounted to a light emitting element by a COG (Chip On Glass) method are all included in a light emitting device.
2. Description of the Related Art
In recent years, as a substitute for a liquid crystal display device (LCD), a light emitting device is suggested which is formed of a display panel in which light emitting elements are arranged per pixel and a peripheral circuit for inputting signals to the panel and which displays an image by controlling light emission of the light emitting element.
A light emitting device using a module which is formed by arranging light emitting elements in matrix has been widely developed, and an organic EL (Electro Luminescence) element having a light emitting layer formed of an organic material is attracting attentions. By using an organic compound which can provide favorable light emission characteristics as a light emitting layer of an EL element, efficiency and life have been advanced to be sufficient for practical use.
Such a light emitting device is typically provided with two or three TFTs (thin film transistors) per one pixel.
FIG. 28 shows a conventional pixel configuration in which two transistors are provided for one pixel. A controlling transistor 2801, a driving transistor 2802, a capacitor 2803, a light emitting element 2804, a data line sig, a scan line gate, and a power source line corn are provided (see Patent Document 1).
Further, FIG. 29 shows a circuit configuration of a conventional pixel in which three transistors are provided for one pixel. A controlling transistor 2901, a driving transistor 2902, an erasing transistor 2903, a capacitor 2904, a light emitting element 2905, a data line sig, a controlling scan line gate1, an erasing scan line gate2, and a power source line corn are provided (see Patent Document 2).
In the circuit configurations of pixels shown in FIGS. 28 and 29, a current supplied to a light emitting element in each pixel, that is, luminance and light emission or non-light emission of a light emitting element in each pixel are controlled by controlling on/off of the transistors.
Even when predetermined luminance is obtained with a current I0 by applying a certain voltage V0, the predetermined luminance cannot be obtained by applying the voltage V0 since luminance characteristics of a light emitting element change with time and only a current I0′ is supplied to the light emitting element.
A light emitting element changes with time because the light emitting element generates heat when voltage or current is supplied thereto, thereby characteristics of an interface between film qualities and an interface between electrodes change. Further, a degradation condition differs between each light emitting element, which causes an image sticking.
Accordingly, a voltage of the opposite direction (a reverse bias voltage) to a voltage applied to a light emitting element when it emits light is applied to alleviate uneven charges of the light emitting elements are regularly, thereby the degradation of the light emitting element is suppressed and reliability thereof is improved (see Patent Document 3).
[Patent Document 1]
Japanese Patent Laid-open No. 2001-343933
[Patent Document 2]
Japanese Patent Laid-open No. 2001-324958
[Patent Document 3]
Japanese Patent Laid-open No. 2001-117534